1. Field of the Invention
The invention pertains generally to magnetic heads, and more particularly to combined read/write magnetic heads.
2. Description of the Related Art
A write magnetic head is a device used to magnetize regions of a magnetic medium, e.g., a magnetic tape, and thereby "write" information onto the magnetic medium. A read magnetic head is a device used to detect magnetized regions on a magnetic medium, and thereby "read" information on the magnetic medium.
Conventionally, a write magnetic head includes a magnetic yoke having a gap exhibiting a relatively high magnetic reluctance, e.g., an air gap. An electric coil encircles the yoke. By passing an electric current through the coil, a magnetic field is produced extending through the yoke. The corresponding fringing magnetic field at the gap is used to magnetize regions of, and thereby write information onto, a magnetic medium, e.g., a magnetic tape, positioned adjacent to, and moving past, the gap.
A conventional read magnetic head is generally similar to the write magnetic head described above. In use, a magnetic medium, e.g., a magnetic tape, is moved past the gap in the magnetic yoke. Magnetic flux, emanating from magnetized regions in the magnetic medium, is coupled into the yoke via the gap. Time-wise variations in magnetic flux induce an output voltage across the coil encircling the magnetic yoke.
Read magnetic heads have been developed in which the output voltage is proportional to magnetic flux, rather than time-wise changes in magnetic flux. Such a read head typically employs a magnetoresistive element (MRE). As is known, when a current is passed through a unidirectionally magnetized MRE, the electrical resistance of the MRE, and therefore the voltage drop across the MRE, is related to the angle between the direction of magnetization and the direction of current flow. If the MRE is exposed to an external magnetic field, e.g., a magnetic field emanating from a magnetic tape, which external field is, for example, oriented transversely to the initial magnetization direction of the MRE, then the magnetization direction will necessarily be changed and the MRE will exhibit a corresponding change in resistance.
In general, the change in resistance of an MRE is a nonlinear function of the field strength of the external magnetic field to be sensed. To achieve a linear response, the MRE is preferably operated so that the angle between the initial magnetization direction and the direction of current flow is 45 degrees. This is accomplished, for example, by magnetizing the MRE along a direction which is at 45 degrees to a longitudinal direction of the MRE and flowing current along the longitudinal direction. Alternatively, the MRE is provided with a barber pole configuration of electrical conductors, i.e., spaced, parallel conductive strips applied to the surface of the MRE. These conductive strips, which constitute equipotential surfaces, are oriented so that electrical current in the MRE flows between the conductive strips at an angle of 45 degrees relative to the longitudinal direction of the MRE, along which the MRE is magnetized. (Regarding barber pole MREs, see U.S. Pat. No. 4,052,748 issued to K. E. Kuijk on Oct. 4, 1977.)
In the case of, for example, magnetic tapes, information is written onto, and read from, spaced, parallel tracks on the tapes. To increase information density, the width of the tracks, as well as the spacing between the tracks, has been steadily reduced. To achieve magnetic heads having correspondingly small dimensions, such heads are now being manufactured using thin-film processing of the type used to manufacture integrated circuits in silicon substrates. The resulting magnetic heads consist of thin layers of magnetically permeable and substantially magnetically impermeable materials.
To achieve track alignment between the read and write magnetic heads used in conjunction with magnetic tapes, it has been proposed that a read head be integrally combined with a write head in a unitary structure. This combined read/write head, fabricated using thin-film processing, is described in C. H. Bajorek et al, "An Integrated Magnetoresistive Read, Inductive Write High Density Recording Head", Published Proceedings of 20th Annual Conference on Magnetism and Magnetic Materials, American Institute of Physics, New York, 1975, pp. 548-549. As shown in FIG. 1, the Bajorek et al combined read/write head 10 is formed on a nonmagnetic substrate 20 and includes a read head consisting of magnetically permeable, permalloy layers 30 and 60 separated by a layer of substantially magnetically impermeable silicon dioxide 40, which defines the read gap of the read head. An MRE film 50 and an overlying permanent magnet film 55 are buried in the silicon dioxide layer 40. The permanent magnet film serves to magnetize the MRE film in a direction which is at 45 degrees to the direction of current flow in the MRE film.
The Bajorek et al combined read/write head 10 also includes a write head consisting of the permalloy layer 60, shared with the read head, an overlying hard gold layer 80, which defines the write gap, and another, overlying permalloy layer 90. Here, the permalloy layers 60 and 90 serve as the poles of the write head. A layer of soft gold 70, buried in the hard gold layer 80, constitutes a one-turn write coil.
Significantly, when writing information onto a magnetic medium 100 with the write head of the combined read/write head 10, the permalloy layers 60 and 30 serve to shield the MRE film 50 from the flux produced by the write head, and from other stray flux. On the other hand, when reading information on the magnetic medium 100 with the read head of the combined read/write head 10, flux emanating from the magnetic medium 100 is not communicated to the MRE film 50 by the permalloy layers 60 and 30 because, as before, these layers serve to shield the MRE film 50 from the flux. Rather, the only flux which reaches the MRE film 50 is that emanating from the magnetic medium 100 which directly impinges upon the MRE film 50.
While the Bajorek et al combined read/write head 10 has advantageous features, it has at least one significant disadvantageous feature. That is, when a magnetic tape is drawn past the read gap, the material of the read gap undergoes wear. Because the MRE film 50 is located in the read gap, it too undergoes wear and a corresponding degradation in sensitivity. This wear of the MRE film 50 generally precludes commercial viability.
A thin-film read head which avoids wear of the MRE is disclosed in Japanese Kokai 63-37811, published on Feb. 18, 1988 and listing Hiroaki Yoda as inventor. As depicted in FIG. 2, the Yoda read head 110 is formed on a nonmagnetic substrate 120 and includes a layer 130 (having an undisclosed composition and function) overlying the substrate 120. An MRE 140 overlies the layer 130, while an electrically insulating layer 150 overlies the MRE 140. It is assumed that the MRE 140 is a single-domain magnetic element (rather than a multi-domain element) and is thus relatively sensitive to the magnetic fields to be sensed.
The Yoda read head 110 also includes a broken (discontinuous) flux guide having sections 160a and 160b, each of which overlies the insulating layer 150 and partially overlaps the MRE 140. An electrically insulating layer 170 overlies the broken flux guide and defines the read gap of the read head 110. An electrical conductor 180, buried within the insulating layer 170, serves to produce a biasing magnetic field for the MRE 140. A continuous flux guide 190 overlies the insulating layer 170.
In the operation of the Yoda read head 110, the flux guides 190, 160a and 160b serve to communicate flux emanating from a magnetic medium 200, e.g., a magnetic tape, to the MRE 140. That is, flux enters the continuous flux guide 190, is communicated to flux guide section 160b, which communicates it to MRE 140, which in turn communicates it to flux guide section 160a.
As is evident from FIG. 2, the MRE 140 of the Yoda read head is located outside the read gap, and is therefore not subject to MRE wear and degradation. Therefore, in light of the teachings of the Bajorek et al article, it would appear that a commercially viable read/write head, in which the MRE does not undergo wear, is readily achieved by combining a write head with the Yoda read head, with the continuous flux guide 190 (see FIG. 2) being common to both the read head and the write head. However, when this is done, new, previously unrecognized, significant problems arise, as discussed below.